This invention relates in general to video displays and in particular to anti-spot burn protection circuitry for a CRT in a video display.
In a video display or other system utilizing a cathode ray tube (CRT) upon which are applied high voltages, adequate protection should be provided to protect the CRT's screen from a continuous beam of undeflected, energetic electrons impinging on the same spot when deflection voltages are removed. These deflection voltages may be removed either upon video display turn-off or upon failure of a low voltage power supply. In either case, permanent damage to the fluorescent coating of the CRT's screen may result. Collapse of the horizontal and vertical sweeps when the video display is either turned-off or deflection power is lost causes the electron beam to be concentrated at the free-fall center of the CRT, or the point where the beam impinges on the screen when undeflected by magnetic fields.
In general, a video display utilizing a CRT includes a low voltage power supply for driving the scanning and receiver circuitry and a high voltage power supply for energizing the accelerating grids of the CRT. The CRT's electron beam current is controlled by the relative potentials on the cathode and control grid electrodes with the cathode being coupled directly to the video input signal. The various electrode grids in the CRT perform functions as brightness control, picture focusing and contrast and background selection. When the video display is turned-off, it is possible, if the potential difference between the cathode and the brightness control grid does not exceed a given voltage value characteristic of the CRT, for electrons from the cathode to continue to be accelerated toward the phosphor-coated faceplate of the CRT long after the scanning voltages have decayed to zero. This undeflected positioning of the electron beam upon the CRT's faceplate produces a stationery bright spot. Since the undeflected electron beam will always impinge on the viewing screen at the free-fall center of the CRT, extended use of the video display results in a burned and permanently damaged spot at the point where the electron beam strikes the viewing screen when undeflected by magnetic fields.
In general, two approaches have been used in attempting to avoid the problem of spot-burn on the CRT of a video display. On approach involves the use of large B+ filter capacitors in maintaining a residual charge to permit the continued operation of the horizontal and vertical deflection circuits following video display turn-off. In this approach the intensity of the electron beam is not manipulated, but rather the residual energy of the electron beam is spread over a large portion of, if not the entire, viewing screen. This technique significantly increases the cost and complexity of the horizontal and vertical drive circuits. The other approach to eliminating this problem involves turning the CRT off by varying the potential difference between the brightness control grid and the cathode. Various attempts to solve the problem of spot-burn utilizing this approach are detailed in the prior art.
One such approach for preventing spot-burn by varying the voltage on a control grid is detailed in U.S. Pat. No. 3,629,645. This approach involves coupling the horizontal blanking pulses to a switching network comprised of a diode and a capacitor in parallel. The switching network is connected between the brightness control and ground, such that when a cessation of negative blanking pulses occurs the capacitor becomes positively charged with the diode becoming reverse biased and non-conducting. This results in the brightness control becoming disconnected from ground with the bias voltages on the control grids permitted to increase in the positive direction and thereby increase the beam current flowing through the picture tube. This causes an increase in beam current and the resulting discharge of stored accelerating energy to occur before the horizontal and vertical scans have collapsed, thereby preventing a stationery bright spot from forming on the viewing screen. While this approach adequately dissipates electron beam energy prior to scan cycle termination it represents an overly complicated solution to this problem. Not only must the flyback transformer be tapped as a source of input pulses to the switching network of this invention, but a neon bulb, or a second properly biased diode, must be incorporated between this switching network and the low voltage power supply to effect reversal of the operating state of the diode in this switching circuit. In addition, a potentiometer, various resistors and a capacitor are necessary for this invention to operate properly in a television receiver.
Another approach to solving the problem of spot-burn is disclosed in U.S. Pat. No. 3,784,870. The invention described therein involves the use of a three-input logic circuit with the three inputs being: a first direct current voltage indicating the presence or absence of horizontal deflection signals, a second direct current voltage indicating the presence or absence of vertical deflection signals, and mixed horizontal and vertical blanking signals for the electron beam. Should a failure occur in either the horizontal or vertical deflection circuits, the resulting change in direct voltage level input causes the logic circuit to become disabled resulting in a bias change on the control grids of the image presentation device so as to blank off the electron beam until corrective repairs are made. The system makes use of a single integrated circuit device for receiving, processing and decision-making functions. This system was designed primarily for use in communication systems employing a storage tube for either the selection of a frame of television information to be transmitted by means of an audio channel or in the recording and recreation of this television information when received by means of a communications link. While of particular value in such a specialized application, this invention would significantly increase video display unit costs particularly in a non-television application such as in a computer terminal or a data display.
Still another approach designed to avoid CRT faceplate spot-burn damage is disclosed in U.S. Pat. No. 3,510,722. This approach makes use of a discharge transistor whose collector to emitter path is in shunt with that of the video output stage. While biased off during normal operation, the discharge transistor is turned on when the television receiver is turned off. Current passing through the discharge transistor drives the cathode of the CRT towards ground thus increasing the forward bias on the electron guns of the CRT. As the cathode voltage becomes more negative, the voltages on the control grid electrodes do also, thus tending to maintain the potential difference between the control grids and the cathode relatively constant. When the control grid to cathode bias reaches a point where substantial beam current starts to flow a high voltage capacitor begins to discharge which loads the horizontal deflection generator. This, in turn, causes the discharge transistor to conduct more heavily causing a further increase in CRT current resulting in the fast discharge of the high voltage supply before the video screen supply is fully decayed and before the deflection raster has collapsed. The beam discharge energy is thus distributed over a relatively large area thereby minimizing the danger of localized damage to the screen. This approach therefore requires another transistor circuit in parallel with the video output stage for proper operation.
The present invention, however, makes use of a minimum number of additional components in combination with existing video output circuitry in providing reliable and safe protection against CRT screen spot-burn in a video display.